1. Field of the Invention
The present invention relates to optical amplifiers, and more particularly to a technique for controlling the gain profile of an optical amplifier equipped with an amplification medium containing rare-earth ions.
2. Description of the Related Art
Wavelength division multiplexing (WDM) communications technology is one of the most effective means for meeting the sharply increasing demand for optical communication. Since WDM increases the communications capacity by increasing signal channels, the available bandwidth for signals must be expanded.
The current WDM communications use rare-earth ion doped optical fiber amplifiers in many cases. One of the bands used for signal amplification is the C band (1530–1570 nm) where the Erbium doped optical fiber amplifier (EDFA) can work effectively.
Meanwhile, the S band (1460–1530 nm) is drawing attention as the next-generation band that has a transmission loss and a low dispersion as low as that in the C band. The Thulium doped optical fiber amplifier (TDFA), of which amplification band lies in the S-band, is under intensive research.
S-Band TDFA has attained a power conversion efficiency (40%) as high as that accomplished by EDFA for use in the L band, and the experiment of its WDM transmission was successful (for example, see OFC2001 PD-1). Furthermore, C-band EDFA is used together with L-band EDFA for band combination to widen the band for signal amplification. Indeed, there is a report that this combination attained a broadband data transmission of 10.9 Tbit/s (for example, see OFC2001 PD-24).
In WDM communications using plural amplifiers equipped with rare-earth doped optical fibers as amplification media, such a problem is posed that the spectrum of output light from the amplifiers changes and occasionally deforms significantly.
This is because the power of input signal light to the optical amplifier fluctuates with time-varying transmission loss and with changes in the number of signal channels, thereby the gain spectrum of the optical amplifier changes, and such changes are accumulated to result in signal deformations.
Since deformations of the gain spectrum in the amplifier become a cause of limiting the transmission distance, the gain spectrum must be held constant. For this purpose, it is effective to control the gain profile of each amplifier to be constant (hereinafter, may be called the gain profile constant control (GPCC)).
As an example of such a control method for silica-based EDFA, there is a method that monitors a gain of a signal light in a channel and controls the power of pump light so that the gain in a channel becomes constant. This is an almost established control technique for maintaining the gain profile constant in silica-based EDFA. The gain profile in fluoride PDFA can be held constant by a similar control method. The amplification medium (optical fiber) of such optical amplifiers operates according to the amplification mechanism in which levels related with the amplification are substantially two. This is a mechanism rarely seen that allows to handle the amplification process only related with the upper level related with the amplification (hereinafter which also refers the amplification upper level) and the lower level involved in the amplification (hereinafter which also refers the amplification lower level) or ground level.
The above mechanism will be described with reference to FIG. 1. FIG. 1A is a schematic diagram illustrating the excitation energy levels in silica-based EDFA, and FIG. 1B is a schematic diagram illustrating the excitation levels in the Praseodymium doped optical fiber amplifier (PDFA).
In the silica-based EDFA, the amplification lower level agrees with the ground level, as shown in FIG. 1A. Thus the energy levels related with amplification are the only two levels—the amplification upper level (4I13/2) and the amplification lower level (4I15/2). By monitoring the gain of a signal right in a channel and adjusting an intensity of pump light source so that the gain in the channel becomes constant, the gain profile can be held constant.
Referring now to FIG. 1B, PDFA has three energy levels related with amplification—amplification upper level, amplification lower level and ground level. In PDFA, however, since the lifetime of fluorescence at the amplification lower level is very short, compared with that at the amplification upper level, the lower level can be neglected. Then it becomes possible to maintain the gain profile constant by the same technique employed in the silica-based EDFA.
When the energy levels related with amplification are substantially two, the gain profile can be relatively easily maintained constant.
In general, however, there are few cases where the energy levels related with amplification can be regarded as only two in optical amplifier using rare-earth ion doped medium. The above method of monitoring the gain at one signal wavelength and maintaining the gain profile unchanged by adjusting the intensity of the pump light based on changes in the gain of the signal is not applicable to common optical amplifiers. In other words, the gain profile cannot be maintained constant against fluctuation of input signal levels and other conditions (for example, changes in temperature) only by adjusting the intensity of pump light to hold the gain at one single wavelength constant.
Thus, in principle, the gains at two different signal wavelengths must be monitored, and the gain control has been complex.
As mentioned above, an optical amplifier used an amplification medium doped with the part of rare-earth ion (an optical fiber or an optical waveguide) can be controlled a gain profile constantly by monitoring a gain at one signal wavelength and controlling an intensity of a pump light. However, an optical amplifier used other rare-earth ion doped amplification medium cannot be controlled a gain profile constantly.
Apart from the above mentioned the gain profile constant control method that the gain at one signal wavelength is monitored and an intensity of a pump light is adjusted based on change thereof, a method for maintaining a gain profile constantly by using oscillator or control light is provided.
As an example of using oscillator, in E. Desurvire et al., “Gain control in erbium-doped fiber amplifiers by all-optical feedback loop”., IEEE, Electronics Letters. Vol. 27, No. 7, pp.560–561, 28th Mar. 1991, the technique is described that the construction of feedback loop is introduced, a laser oscillation is caused at one wavelength of ASE, the gain at the wavelength is clumped, and thus gain spectrum is controlled. In addition of the literature, the techniques of clumping a gain includes EP 0 497 491, Japanese Patent Application Laid-open No. 9-509012(1997), and Japanese Patent Application Laid-open No. 11-145533(1999).
As an example of using control light, E. Desurvire et al. (IEEE, Photonics technology letters, vol. May 1991 entitled Dynamic gain compensation in saturated erbium-doped fiber amplifiers.) discloses that ASE level at one wavelength is monitored and a power of control light is controlled so that the ASE level becomes constant.
Japanese Patent Application Laid-open No. 2000-261079 discloses the optical amplifier and the controlling method that a power of control light which is inputted from back-side of a rare-earth ion doped optical fiber and outputted from the optical fiber is monitored and a control light source is controlled so that the power of control light becomes constant.
Japanese Patent Application Laid-open No. 8-304856(1996) discloses that a pump power inputted to a amplification medium and a pump power outputted from the amplification medium are monitored and a power of a control light is controlled based on a ratio between both pump powers.
Further, it is also disclosed that a total input power of a control light and signal light are monitored and a power of the control light is controlled so that the total input power becomes constant, and a total output power of a control light and signal light are monitored and a power of the control light is controlled so that the total output power becomes constant.
The methods of using oscillator or control light as described above can control the gain spectrum of silica-based EDFA wherein the levels greatly related to amplification (i.e. it refers the level that should be considered for determining the state of the gain spectrum) are only two, but they cannot control TDFA and other rare-earth ion doped optical amplifiers.
Thus it has been desired that an optical amplifier and a control method which can be applicable to the optical amplifier used the common rare-earth doped amplification medium and control the gain profile constant by easy method.